This paper discusses an ontology based clinical decision support system for the specialty of Geriatric Medicine. We created a domain level ontology based on Handbook of Geriatrics and then mapped it to an upper level Basic Formal Ontology. The decision support system has been developed in Prolog. For accessing this ontology, we created an interactive web and android application which acts as a front-end to the system. Both applications are able to display the ontology structure and to predict the disease based on the symptom entered. We have uploaded our ontology to Bio-Portal, which is one of the most comprehensive biomedical ontology repository. It can be accessed using the URL https://bioportal.bioontology.org/ontologies/G-O.